1. Field of the Invention
The present invention relates to a lithographic apparatus and a positioning device, which can be used in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Such a lithographic apparatus may further include one or more positioning devices for displacing and positioning an object table provided with, e.g., the substrate or the patterning device.
Drive units such as linear motors, planar motors or linear actuators are often applied in the positioning device for generating the desired force. Examples of such positioning devices including linear motors can, for example, be found in U.S. Pat. No. 5,815,246 and U.S. Pat. No. 5,969,441. In general, such drive units or motors include a first part that includes an array of permanent magnets (also referred to as the magnet array) that extends in a first direction and a second part that includes a plurality of coils (also referred to as the coil assembly), positioned adjacent to each other in the first direction. During operating, the coils are supplied with an electric current (e.g. from a three-phase current supply) thus generating an electromagnetic force between the first part and the second part of the motor.
In general, some type of bearing may be provided between the first part and the second part allowing both parts to be displaced relative to each other in the first direction. Additionally, the bearing may be arranged to maintain a predefined distance between the first and second part in a second direction substantially perpendicular to the first direction. Examples of such bearings are gas bearings (such as air bearing), ball bearings or slide bearings. In case of an air bearing (or any other gas bearing), a pre-load force is often applied. This pre-load force may be provided by, e.g., the weight of the moving assembly (in case the second direction corresponds to the vertical direction) or may be provided by a separate unit including, for example, a ferromagnetic member attached to the first part and a permanent magnet member attached to the second part.
In a lithographic projection apparatus, such a pre-loaded air bearing is often applied since it allows a displacement that is substantially frictionless and contactless. The latter is beneficial in that it may reduce the risk of contamination (by particles) that could occur in other types of bearings such as ball bearings. Such an air bearing usually extends along the first direction or includes various so-called bearing pads that are arranged along the first direction. In such an arrangement, the repelling force of the air bearing combined with the attractive pre-load force provides an equilibrium force at certain value of the distance between the first part and the second part in the second direction.
In many apparatuses, e.g. a lithographic apparatus, the positioning devices are designed as light as possible, in order, for example, to improve the dynamic behavior of the apparatus and to increase the productivity yield.